Vehicle traction control system may be used to control or limit wheel drive torque and thereby reduce wheel slip in order to provide improve vehicle handling. However, one challenging tasks for current traction control systems occurs in a situation where a vehicle may be stuck in deep snow, mud, or sand. Under such conditions, even with reduced and controlled wheel torque, the particular vehicle position and road conditions create a situation where there is simply insufficient friction to generate sufficient vehicle motion to enable the vehicle to negotiate out of the stuck condition.
As such, under such conditions, a driver may manually disable the traction control system and proceeds to rock the vehicle longitudinally by manually changing the transmission sequentially between forward and reverse. For example, in an automatic transmission having gears such as Park, Reverse, Neutral, Driver, and Low (PRNDL), the driver manually oscillates between D and R. Many times, despite the driver's best intention and effort, it is still not possible to dig the vehicle out of the ditch or other stuck condition.
One approach to increase a tractive effort of the vehicle is described in WO 0035688. In this example, the vehicle is oscillated vertically via a hydraulic suspension-like actuator to get out of sand.
However, the inventors herein have recognized a disadvantage with such an approach. Specifically, there is no coordination between the vertical force and motion of the vehicle, such as the longitudinal action of the vehicle. As such, if the vehicle operator is unable to synchronize the vehicle motion with the vertical/suspension force, the vehicle may actually become further entrenched in the sand.
To assist the vehicle operator in negotiating stuck vehicle and other situations, the inventors herein have recognized that coordination between forces generated by a suspension system and forward-backward vehicle motion may be used to increase vehicle tractive effort.
For example, active suspension intervention may be coordinated with sequential or oscillatory vehicle forward-reverse rocking action to increase vehicle traction. In one embodiment, the suspension actuation may be used to increase an active force of the suspension at appropriate instances or durations so that the amplitude of the resulting vehicle oscillation increases. This increase may then eventually help the vehicle to come out of it stuck condition, or hole.
Thus, the inventors herein have recognized that action similar to what a child does on a swing, where the child modulates the lower part of his legs to increase the swing motion, may be applied to the stuck vehicle condition via an active suspension, for example.